Off-chip drivers are known for carrying out various tasks. For example, off-chip drivers are used where a signal which has been produced within a chip, i.e., within an integrated semiconductor circuit (also referred to as a module) is intended to be amplified before being transmitted to a circuit, which is external to the chip. The signal power or the signal level of the signal, which is produced on the chip, can be matched to the requirements of the external circuit without the entire integrated semiconductor circuit supplying the signal power or signal level.
An off-chip driver can be used to terminate lines, which lead from an external circuit to the chip, i.e., the external lines present a terminating impedance, whose impedance value ensures that the signal profile via the lines is as clean as possible.
Since an off-chip driver forms an interface between an integrated semiconductor circuit and an external circuit, it is normally formed on the chip, i.e., as part of the integrated semiconductor circuit. Its designation as an off-chip driver indicates the fact that an off-chip driver is normally used to drive a signal intended for the exterior (outside the chip).
FIG. 1 shows a schematic illustration of a conventional application configuration of an off-chip driver. The illustration shows an integrated semiconductor circuit 100, which, for example, includes a logic circuit 10, a driver circuit arrangement 20, which acts as the off-chip driver, and a driver calibration circuit arrangement 40. Such an integrated semiconductor circuit 100 is normally encapsulated in an airtight package.
Connecting pins 101, which project from the package, are used to provide an electrical connection for circuits that are located outside the package. The illustrated integrated semiconductor circuit 100 has 11 connecting pins 101, for example. These are power-supplying connecting pins Vdd and ground, the signal-carrying connecting pins S1-S8 as well as the connecting pin ZQ, which connects to a reference impedance Zref. Such a reference impedance may also, of course, be provided within the package, for example, as part of the integrated semiconductor circuit 100.
This is worthwhile, for example, when the impedance to be represented by the reference impedance is already known at the time when the integrated semiconductor circuit 100 is being designed.
Although this is not shown explicitly in the figure, it is obvious to those skilled in the art that each of the connecting pins is connected to at least one corresponding line on the integrated semiconductor circuit 100. For example, the illustration shows that both the logic circuit 10 and the driver circuit arrangement 20 as well as the driver calibration circuit arrangement 30 are connected to Vdd and to ground.
In the illustrated integrated semiconductor circuit 100, the logic circuit 10 represents the core of the semiconductor circuit 100, where data is, for example, processed, stored, or produced. The data flows to and from the logic circuit 10 via signal lines 11, which are connected to the driver circuit arrangement 20. The driver circuit arrangement 20 acts as an interface between the illustrated 8 signal lines 11 and the 8 signal lines 23, which are individually annotated S1-S8, and lead to corresponding connecting pins 101 on the semiconductor circuit 100. The signal lines S1-S8 are connected via the connecting pins 101 to corresponding signal lines 41 from an external circuit 40 which, for example, represents a circuit that uses the logic circuit 10. For example, the integrated semiconductor circuit 100 together with the logic circuit 10 could be a memory chip, and the external circuit 40 could be a computer motherboard.
The operating behavior of the driver circuit arrangement 20 can be influenced via the driver calibration circuit arrangement 30, which is connected via, for example, 5 signal lines 24 to the driver circuit arrangement 20. The method of operation of a driver calibration circuit arrangement and its influence on an off-chip driver will be described in a general form in the following text, i.e., without reference to the figures.
High-quality off-chip drivers can operate with a specific and stable driver impedance. This is frequently achieved by digital compensation algorithms, which digitally vary the power of the off-chip driver in order to match the driver power to an intended value.
Since integrated semiconductor circuits are normally designed, manufactured, and marketed as individual components without any knowledge of the signal and/or impedance requirements of any external circuit, mechanisms which allow the off-chip driver to be matched to the signal and/or impedance requirements of an external circuit once the chip and the external circuit have been connected to one another are desirable.
Such matching of an off-chip driver is normally carried out based on three parameters, i.e., based on a first supply potential, a second supply potential and a reference impedance, which can be applied to or connected to specific connecting pins of the chip in a specific configuration. The impedance of the off-chip driver is varied as appropriate based on these parameters.
According to the prior art, the off-chip driver can be matched by a (digital) search algorithm. A variable impedance of the off-chip driver forms a voltage divider between the two supply voltages via a series circuit with the reference impedance and compares an output voltage from the voltage divider with an internally produced reference voltage, which is equal to the mid-value between the two supply potentials. The variable impedance of the off-chip driver is varied in accordance with the search algorithm until the output voltage from the voltage divider and the reference voltage are identical, with sufficient accuracy. If the voltage divider includes a series circuit formed by the variable impedance of the off-chip driver and the reference impedance, the variable impedance at the end of the search algorithm is equal to the reference impedance, with a specific accuracy.
Such matching of the off-chip driver has been found to be disadvantageous in many applications, inter alia as an interface between DDR memory modules (DDR=“double-data-rate”) and corresponding external circuits. In particular, an off-chip driver that has been matched in this way exhibits undesirable distortion of the slew rate and thus of the operating cycle in a circuit combination such as this.
A driver system, a driver calibration circuit arrangement for calibration of an impedance of a driver circuit arrangement, and a method for calibration of an impedance of a driver circuit arrangement, which can improve driver behavior, is desirable.